Hail size discrimination for polarimetric WSR-88D radars

Use of dual-polarization radar data in the discrimination of precipitation types has been demonstrated successfully. The current NEXRAD Hydrometeor Classification Algorithm (HCA) developed by the National Severe Storms Laboratory discriminates between ten hydrometeor categories, including the mixture of rain and hail. However, the scattering characteristics of hailstones vary widely across the spectrum of sizes and shapes, and are significantly dependent on the amount of melting that has occurred. The most substantial societal impact comes from those hailstones considered severe (diameter D > 2.5 cm in diameter) by the National Weather Service, as such large hail inflicts the most damage to property. Therefore, it is desirable to develop a modification to the current HCA to discriminate between small (D < 2.5 cm), large (2.5 cm < D < 5.0 cm), and giant (D > 5.0 cm) hail, which will aid in the detection, discrimination, and warning of severe hail events.

This paper explores the practicality of such a hail size discrimination based on observations, modeling, and theoretical computations. The result is a novel set of rules used to discriminate between small, large, and giant hail, taking into account the height of the hailstones relative to the melting layer. The algorithm is applied to numerous datasets, including recent cases with extensive ground truth.